Most load-carrying trailers which are towed behind a vehicle usually have a trailer bed or frame mounted at a fixed height on supporting wheels so as to have sufficient clearance to negotiate the rough ground over which the trailer may be hauled. Trailers of this type can be difficult to load and unload.
In some trailers, commonly called drop-bed trailers, the wheels are mounted for vertical movement relative to the trailer bed so that the bed, or at least the rearward end, thereof, can be raised and lowered at will to facilitate loading and unloading. However, facilitating relative movement of the wheels and trailer frame, while at the same time providing a useful trailer suspension has proven elusive.
One common type of prior art drop-bed trailer employs a robust U-shaped axle, with the wheel being mounted outboard the curled ends, and the frame mounted by leaf springs or the like to the axle. The suspension afforded by this type of trailer is quite adequate, and the trailer is quite robust. However, the bed of this trailer is difficult to raise and lower on uneven ground.
Another common type of prior-art trailer includes independently pivotal, wheel carrying support arms, connected to the trailer frame during towing by a half leaf spring, and each pivotal by a respective cable winch. The half leaf spring of this type provides for a relatively rough ride, as well the dual cable winches make one-person operation of this trailer inconvenient.
Another from of prior-art trailer includes independent wheel carrying elongated members connected to the trailer frame during towing by an assortment of roller guide pins and receivers, adjusting bolts and complicated alignment of the elongated members to the frame. The trailer was operated by a cable pulley system; cable slack is required to allow the suspension system to compress, lack of controlled sufficient amount of cable slack restricted the compression of the suspension system, resulting in damage to the pulley sheaves and cable, causing metal fatigue and failure to the main suspension bolt rendering the trailer, inoperable.